Data communication between computer systems for applications such as web browsing, electronic mail, file transfer, and electronic commerce is often performed using a family of protocols known as IP (Internet protocol) or sometimes TCP/IP. As applications that use extensive data communication become more popular, the traffic demands on the backbone IP network are increasing exponentially. It is expected that IP routers with several hundred ports operating with aggregate bandwidth of Terabits per second will be needed over the next few years to sustain growth in backbone demand.
As illustrated in FIG. 1, the Internet is arranged as a hierarchy of networks. A typical end-user has a workstation 22 connected to a local-area network or LAN 24. To allow users on the LAN to access the rest of the Internet, the LAN is connected via a router R to a regional network 26 that is maintained and operated by a Regional Network Provider or RNP. The connection is often made through an Internet Service Provider or ISP. To access other regions, the regional network connects to the backbone network 28 at a Network Access Point (AP). The NAPs are usually located only in major cities.
The network is made up of links and routers R. In the network backbone, the links are usually fiber optic communication channels operating using the SONET (synchronous optical network) protocol. SONET links operate at a variety of data rates ranging from OC-3 (155 Mb/s) to OC-192 (9.9 Gb/s). These links, sometimes called trunks, move data from one point to another, often over considerable distances.
Routers connect a group of links together and perform two functions: forwarding and routing. A data packet arriving on one link of a router is forwarded by sending it out on a different link depending on its eventual destination and the state of the output links. To compute the output link for a given packet, the router participates in a routing protocol where all of the routers on the Internet exchange information about the connectivity of the network and compute routing tables based on this information.
In recent years the volume of Internet traffic has been quadrupling each year. At the same time, the speed of the optical links that carry this traffic has been increasing at a slower rate, quadrupling every three years. Thus, to keep up with traffic demands, networks have added multiple links or trunks between network access points to scale bandwidth at a rate faster than the increase in individual link bandwidth. These multiple trunks may be transmitted on separate fibers or as separate channels wave-division multiplexed over a single fiber, or both.
Wavelength-division multiplexing (WDM) is an approach to increasing bandwidth between NAPs by multiplexing several channels on a single fiber. With this approach an existing fiber between two NAPs, which originally carried a single channel, is enabled to handle a number (typically 20) channels of the same rate. To accomplish this, a WDM multiplexer is used to combine several channels by modulating each with a slightly different optical wavelength or color of light. The channels, each at a different wavelength, are then combined into a single optical fiber. At the far end of the fiber, separating out the different colors of light demultiplexes the channels. Upgrading one or more fibers to WDM results in large numbers of parallel trunks between NAPs.